In an orthogonal acceleration Time of Flight mass spectrometer an analogue signal is created when ions are detected by an ion detector. Each analogue signal which corresponds with a single time of flight transient (i.e. the signal due to a single pulse of ions being detected by an ion detector which comprises an electron multiplier and wherein the signal is the result of a single pulse of ions being orthogonally accelerated into a time of flight or drift region of the Time of Flight mass spectrometer) is transmitted via a signal line to a data recording device such as an analogue to digital converter (“ADC”). The signal received by the analogue to digital converter is then digitised by the analogue to digital converter (“ADC”) and the resulting digitised signal may then be further processed before being summed into memory together with other digitised signals corresponding to subsequent time of flight transients.
The signal line from the ion detector to the analogue to digital converter (“ADC”) may include a number of different components including co-axial cables, co-axial connectors, co-axial elbows, AC coupling devices, amplifiers, electrical discharge protection devices and attenuators.
It is known that small impedance mismatches in the signal line can give rise to reflections of the signal or echoes. Complex signal paths may comprise many small impedance mismatches and these reflections can appear as perturbations of the baseline signal which may appear in the baseline signal after the main ion signal pulse.
Perturbations in the baseline signal which appear after the main ion signal pulse may also be caused by parasitic capacitance effects which result in ringing effects appearing in the signal after the main ion signal pulse.
For given impedance mismatches the amplitude of the perturbations is directly proportional to the amplitude of the original signal.
In Time of Flight mass spectrometry these undesired perturbations in the baseline signal can result in unwanted artefacts appearing in a resultant mass spectrum or mass spectral data. Such undesired effects can be particularly problematic at high or relatively high ion flux.
EP-1505631 (Hidalgo) discloses a mass spectrometer comprising an ion accelerator, an ion detector and a finite impulse response (“FIR”) filter. The output of the finite impulse response filter is a single value that exits the filter at a time delay which is characteristic of the flight time of the ions. The magnitude of the value which is output by the filter is proportional to the number of electrons generated by the ions having that flight time.
U.S. Pat. No. 6,680,476 (Hidalgo) discloses using a finite impulse response (“FIR”) filter to match the shape of a mass peak.
WO 03/006949 (Youngquist) discloses using a digital filter comprising a finite impulse response (“FIR”) filter, wherein the filter has a Gaussian filter function so as to match the shape of an ion peak.
US 2013/0168546 (Micromass) discloses using a finite impulse response (“FIR”) filter comprising a single or double differential filter or a sharpening filter to extract information relating to the ion arrival time and intensity. The finite impulse response filter may be used to improve the identification and sharpening of ion peaks.
WO 2007/140327 (Waters) discloses using a finite impulse response (“FIR”) filter for smoothing and differentiating signal peaks. The filter may have a Gaussian profile.
It is desired to provide an improved method of mass spectrometry.